A noninvasive technique for measuring local cerebral blood flow (LCBF) by xenonenhanced x-ray transmission computed tomography (CT) has been developed and reported quite extensively in recent years. In this method, nonradioactive xenon gas is inhaled and the temporal changes in radiographic enhancement produced by the inhalation are measured by sequential computed tomography. Time-dependent xenon concentration within various tissue segments in the brain are used to derive both local partition coefficient (Lambda) and LCBF. An assessment of this method reveals that although it has some distinct limitations, it provides functional mapping of blood flow with excellent anatomic specificity. While significant progress has been made in recent years in the understanding of the basic advantages and limitations of this methodology, various aspects of the derivational methodologies used are yet to be optimized and a comprehensive validation of this technique is yet to be completed. The main thrust of this continuation proposal is the comprehensive validation of the technique in critical low-flow states. While the proposed study will be carried out with non-human primates (Papio cynocephalus [anubis]), questions which concern possible future clinical use of such techniques will be addressed in detail. In this study protocols will be tested and optimized for clinical applications and results will be compared to those obtained by other well-established diffusible indicator methods, in particular, in critical low-flow states.